Method for Producing Ti or Ti Alloy, and Pulling Electrolysis Method Applicable Thereto

ABSTRACT

In producing Ti or a Ti alloy through reduction by Ca, an electrolytic-bath salt taken out from a reduction process is electrolyzed to recover Ca and the electrolytic-bath salt as a solid substance, and the recovered Ca and electrolytic-bath salt are delivered to the reduction process. Therefore, heat generation is suppressed in the reduction process by utilizing latent heat of fusion possessed by the solid substance, thereby largely improving production efficiency and thermal efficiency. Additionally, a reaction temperature is easily controlled, and a raw-material loading rate can be enhanced to efficiently produce Ti or the Ti alloy. At this point, using a pulling electrolysis method of the invention, the solid-state Ca and electrolytic-bath salt can be obtained at a low voltage and high current efficiency, i.e., with the relatively small power consumption. When the solid-state Ca and electrolytic-bath salt is used as a Ca source in producing Ti or the Ti alloy through reduction by Ca, the Ti or Ti alloy can efficiently be produced.

TECHNICAL FIELD

The present invention relates to a method for efficiently producing Ti or a Ti alloy at low costs and a pulling electrolysis method in which Ca applicable to reduce Ti and other hard-to-reduce metals can be obtained as a solid-state Ca and an electrolytic-bath salt.

BACKGROUND ART

Metals such as Ti, Zr, Ta, Hf, and V each is a useful metal having excellent properties. These metals are hard to be reduced and it is necessary to separate coexisting homologous elements and impurities. Therefore, usually oxides or halides thereof are formed after the metals are refined through many processing steps, and the metals are produced by reducing the oxides or halides with a metal such as Mg and Na having strong reduction power.

Among others, a metallic Ti and a Ti alloy are excellent in corrosion resistance and design, and further light in weight and excellent in mechanical properties. Therefore, the metallic Ti and Ti alloy are widely used as aircraft materials, heat exchanger materials, chemical plant materials, roof materials, golf-club heads, and the like. Particularly, in recent years, the metallic Ti and Ti alloy are used in instruments of medical fields because of a nontoxic metal for a human body, and the application of the metallic Ti and Ti alloy is being increased.

However, because the metallic Ti is difficult to smelt, the metallic Ti is extremely expensive as a metal, development of a method of producing the metallic Ti with high productivity at low costs is demanded in an industrial scale. That is, in the conventional metallic Ti smelting method, titanium tetrachloride (TiCl₄) is reduced to obtain the metallic Ti by the metal such as Mg and Na having the strong reduction power (reducing metal). However, in the conventional method, because the production is performed in a batch manner, there is a problem in that the improvement in productivity is limited.

For example, in a Kroll process which is of a typical method of industrially producing the metallic Ti, molten Mg is loaded in a reactor vessel, a TiCl₄ liquid is fed from above a liquid surface thereof, and the metallic Ti is produced by reducing TiCl₄ with Mg near the liquid surface of the molten Mg. However, in the Kroll method, a feed rate of TiCl₄ is restricted because the reaction is generated near the liquid surface of the molten Mg in the reactor vessel.

Additionally, Ti granules produced are aggregated because of an adhesion property between Ti and the molten Mg, and the Ti granules are sintered to grow in sizes by the heat of the molten liquid. Consequently, it is difficult to take out the produced Ti to the outside of the reactor vessel, it is also difficult to continuously produce Ti, the improvement in productivity is limited to thereby increase production costs, and a product price becomes extremely high.

Therefore, various researches and developments have been conducted on the Ti production method except for the Kroll process. For example, U.S. Pat. No. 2,205,854 discloses that Ca can be used as a reducing agent for TiCl₄ in addition to Mg. Ca has a stronger affinity for Cl than that of Mg, and Ca is suitable in principle for the reducing agent for TiCl₄. However, the metallic Ti production method in which TiCl₄ is reduced with Ca is not put to practical use yet. This is because that CaCl₂ is hardly electrolyzed.

Another Ti production method includes an Olson process disclosed in U.S. Pat. No. 2,845,386. The Olson process is a kind of oxide direct-reduction process in which TiO₂ is directly reduced by Ca without going the route of TiCl₄. Although the oxide direct-reduction process has high efficiency, the oxide direct-reduction process is not suitable for the high-purity Ti production at low costs. This is because it is necessary to use expensive high-purity TiO₂.

That is, in the Ti production methods disclosed in U.S. Pat. Nos. 2,205,854 and 2,845,386, unfortunately it is not easy to refine Ca, and it is difficult to handle Ca because it is easily oxidized. Additionally, in the oxide direct-reduction process, there is the problem in that it is necessary to use the expensive high-purity TiO₂. Therefore, the Ti production methods disclosed in U.S. Pat. Nos. 2,205,854 and 2,845,386 are not put to practical use yet.

However, Ca has the stronger affinity for Cl than that of Mg, and Ca is suitable in principle for the reducing agent for TiCl₄. When Ca is obtained at low costs, Ca can be used as the reducing agent for Ti and hard-to-reduce metals such as Zr and Hf, and further Ta and V, any of which is caused to form chloride and then reduced by Mg, thus proving to be industrially-useful.

Currently, the metallic Ca is mainly produced by a vaporization reducing method using a carbonated salt as a raw material. However, a Germany technical document (“HANDBUCH DER TECHNISCHEN ELEKTRO CHEMIE” DRITTER BAND (1934) p. 128 to p. 164 “Calcium, Strontium, Barium.” Von Dr. V. Makow) reports that, in early stage in which the metallic Ca was industrially produced, the molten CaCl₂ was electrolyzed and Ca was produced by separating the attached salt by re-melting.

However, a voltage becomes extremely high during the electrolysis in the Ca production by the electrolysis of the molten CaCl₂, described in the Germany technical reference. Therefore, it can be predicted that the required electric power (current×voltage) becomes large, huge electric energies are consumed, and the production costs are increased.

DISCLOSURE OF THE INVENTION

In view of the foregoing, a first object of the invention is to provide a method of producing Ti or the Ti alloy through reduction by Ca, particularly a method of efficiently producing Ti or the Ti alloy at low costs. A second object of the invention is to provide a pulling electrolysis method in which Ca applicable for reducing Ti can be obtained at low electrolytic bath voltage and high current efficiency in order to be applied to the method of producing the hard-to-reduce metal, particularly Ti or the Ti alloy.

The inventors focus on and study the method of reducing TiCl₄ by Ca in order to solve the above problems. As a result of the study, the inventors establish a method (hereinafter the method is referred to as “the method of producing Ti or the Ti alloy through reduction by Ca” with inclusion of various examples), wherein the CaCl₂-containing molten salt in which Ca is dissolved is retained in the reactor vessel, the metallic chloride containing TiCl₄ is caused to react with Ca in the molten salt to generate Ti granules or Ti alloy granules in the molten salt, and the Ti granules or Ti alloy granules generated in the molten salt are separated and recovered from the molten salt, whereby continuously producing Ti or the Ti alloy. Additionally, the inventors propose a pulling electrolysis method enabling to obtain a solid-state Ca applicable to the method. The development background and the obtained findings will be described for each method while the one is “the method of producing Ti or the Ti alloy through reduction by Ca” and the other is “the pulling electrolysis method”.

(Method of Producing Ti or Ti Alloy Through Reduction by Ca)

In the method of producing Ti or the Ti alloy through reduction by Ca, CaCl₂ which is of a by-product in association with the Ti generation is taken out to the outside of the reactor vessel and electrolyzed, and the generated Ca can be used in the reaction for generating the metal such as Ti in the reactor vessel. In this case, one of large advantages of the method is in that rigorous separation of Ca and CaCl₂ is not required.

According to the method of producing Ti or the Ti alloy through reduction by Ca, the Ti generation reaction proceeds in the molten CaCl₂. Accordingly, compared with the Kroll process in which the reaction field is limited to the proximity of the liquid surface because TiCl₄ is fed to a liquid surface of the reducing agent (Mg) in the reactor vessel, the reaction field is enlarged and the heat generation region is also spread to easily perform the cooling, so that the method holds promise of largely enhancing a feed rate of TiCl₄ which is of the raw material for Ti to largely improve productivity.

However, because the reaction of Ca and TiCl₄ in the molten CaCl₂ is an exothermic reaction, in order to maintain the high productivity, it is necessary to cool the heat generation region to dissipate the heat. Therefore, the production efficiency is lowered and the thermal efficiency is also lowered because of the large heat loss.

In the case where the heat rapidly generated due to the excessive TiCl₄ feed rate exceeds the cooling capacity, the reaction field (region where the reaction of Ca and TiCl₄ is generated) gets to an excessively high temperature and the reactor vessel is severely worn. On the contrary, when the temperature is excessively lowered, the reaction rate is decreased. Therefore, in the TiCl₄ reduction process, it is necessary to finely control the temperature to maintain the high productivity.

In order to solve the problem, the inventors further study measures to overcome the difficulty in the reaction-field temperature control while suppressing the decrease in production efficiency and energy loss caused by repetition of the heat generation and heat dissipation as much as possible. As a result, the inventors have an idea in which the molten CaCl₂ solution is electrolyzed while taken out from the reactor vessel and the generated Ca is not returned into the reactor vessel along with the molten CaCl₂ solution (i.e., as the CaCl₂ solution whose Ca concentration is increased), but Ca and the molten CaCl₂ solution are recovered in the form of the solid substance and the solid substance is returned to the reduction process.

The realization of the method can absorb the heat generated by the reaction of Ca and TiCl₄ by utilizing latent heat of fusion possessed by the solid substance containing Ca and CaCl₂. Therefore, the thermal efficiency is largely improved, the reaction temperature is easily controlled, and the method of producing Ti or the Ti alloy through reduction by Ca can be performed further efficiently.

The Germany technical reference reports that a rod-shape Ca is obtained by the electrolysis of the molten CaCl₂.

FIG. 1 is a view showing a schematic configuration of a main part of a calcium furnace for producing Ca by the electrolysis of the molten CaCl₂, disclosed in the Germany technical document. In the calcium furnace (electrolytic apparatus) having the schematic configuration shown in FIG. 1, a calcium chloride (CaCl₂) 16 which is of an electrolyte is loaded in a graphite crucible 15 (cooled steel vessel coated with a graphite plate), and is melted and heated. A part of the electrolyte is solidified to form a solidified electrolyte 20 by cooling in a bottom portion and an inner wall portion of the crucible 15.

Then, electricity is turned on between an anode (positive electrode) 17 and a cathode (negative electrode) 18 to perform the electrolysis. At this point, the cathode 18 is pulled such that variations in current and voltage are decreased according to a degree of depositing Ca on the cathode 18, and Ca 19 is grown in a rod shape. Because the solidified salt adheres to the surface of the calcium rod, the solidified salt is re-melted and separated in the calcium chloride. The Germany technical reference reports that, during the electrolysis, a cathode current density is set to 125 A/cm², the voltage ranges from 35 to 40V, and purity of the re-melted metallic Ca ranges from 98 to 99%.

Although Ca is generated by the electrolysis in the Germany technical reference, the inventors perform experiments and studies on the recovery of the solid substance containing Ca and the electrolytic-bath salt (CaCl₂ is used) in the electrolysis process. As a result, the inventors obtain the following findings: Ca generated by the electrolysis is deposited on the cathode surface by gradually pulling the cathode during the electrolysis, and a phenomenon in which CaCl₂ adheres as solidified substance near the deposited Ca is repeatedly generated to obtain the solid substance in which the Ca and the electrolytic-bath salt are mixed.

(Pulling Electrolysis Method)

In order to develop a pulling electrolysis method of being able to obtain the hard-to-reduce metal, particularly Ca usable in the Ti reduction, the inventors conducts comprehensive study for finding an electrolysis condition that, using the electrolytic-bath salt containing CaCl₂, the tank voltage (hereinafter simply referred to as “voltage”) is decreased while the high current efficiency (i.e., high Ca recovery efficiency) is obtained during the electrolysis. As a result, the inventors obtain the following findings for the bath temperature, the cathode current density (hereinafter referred to as “cathode current density” or simply “current density”), and the cathode pulling rate.

(a) Because the voltage is increased when the cathode current density is increased, in order to decrease the voltage, the cathode and the anode are brought close to each other to shorten a distance between the electrodes (inter-electrode distance), whereby decreasing the current efficiency. On the contrary, when the inter-electrode distance is lengthened, the current efficiency is improved while the voltage is increased. That is, it is difficult to balance the decrease in voltage with the improvement of the current efficiency.

(b) When the electrodes are brought close to each other to decrease the voltage while the current density is set within a predetermined range (0.1 to 200 A/cm²), the inter-electrode distance becomes not more than 7 cm and the voltage becomes not more than 10V.

(c) When the cathode pulling rate is enhanced while the condition (b) is satisfied, the current efficiency is increased. This effect is observed when the pulling rate is not less than 0.05 cm/min. The solid-state “Ca and electrolytic-bath salt” containing Ca generated (deposited) by the electrolysis and the electrolytic-bath salt which adheres to the surface of Ca and is solidified is recovered by pulling the cathode.

(d) When the bath temperature is raised under the condition (c), a Ca concentration in the recovered Ca and electrolytic-bath salt is increased. In the case of the high-temperature electrolytic-bath salt, although the current efficiency is slightly decreased, the current efficiency is increased by enhancing the cathode pulling rate. The super current efficiency is obtained in the case where the pulling rate satisfies the following equation (1):

V≧0.0035×t−2.4  (1)

where V: cathode pulling rate (cm/min), and

t: bath temperature (° C.).

The following unique advantage is generated by being able to recover Ca in the form of the solid-state “Ca and electrolytic-bath salt”.

As described above, instead of the Kroll process in which the continuous production is hardly performed, the inventors establish the method, wherein the CaCl₂ containing molten salt in which Ca is dissolved is retained in the reactor vessel, the metallic chloride containing TiCl₄ is caused to react with Ca in the molten salt to generate the Ti granules or the Ti alloy granules in the molten salt, and the Ti granules or the Ti alloy granules are separated and recovered from the molten salt to continuously produce Ti or the Ti alloy.

As described above, in the production method, CaCl₂ which is of the by-product in association with the Ti generation is taken out to the outside of the reactor vessel and electrolyzed, and the generated Ca can be used in the reaction for generating the metal such as Ti in the reactor vessel. In this case, one of the large advantages of the method is in that rigorous separation of Ca and CaCl₂ is not required.

According to the method of producing Ti or the Ti alloy through reduction by Ca, the Ti generating reaction proceeds in the molten CaCl₂. Accordingly, compared with the Kroll process (reaction field is limited to the neighbor of the Mg liquid surface), the region where the reducing reaction is generated (i.e., reaction field) is remarkably enlarged and the heat generating region is also spread to easily perform the cooling, so that the TiCl₄ feed rate can largely be enhanced to remarkably improve the productivity.

However, because the reaction of Ca and TiCl₄ in the molten CaCl₂ is the exothermic reaction, in order to maintain the high productivity, it is necessary to cool the heat generating region to dissipate the heat. Therefore, the thermal efficiency is also lowered because of the large heat loss.

In the case where the heat, rapidly generated due to the excessive TiCl₄ feed rate, exceeds the cooling capacity, the reaction field becomes an excessively high temperature, and the reactor vessel is severely worn. On the contrary, when the temperature is excessively lowered, the reaction rate is decreased. Therefore, in the TiCl₄ reduction process, it is necessary to finely control the temperature to maintain the high productivity.

In order to solve the problem, the inventors further study the measures to overcome the difficulty of the reaction-field temperature control while suppressing the decrease in energy loss caused by repetition of the heat generation and heat dissipation. As a result, the inventors hit on an idea, in which the molten CaCl₂ solution is electrolyzed while taken out from the reactor vessel, the generated Ca and the molten CaCl₂ solution are recovered in the form of the solid substance, and the solid substance is returned to the reduction process.

The realization of the method can absorb the heat generated by the reaction of Ca and TiCl₄ by utilizing latent heat of fusion possessed by the solid substance containing Ca and CaCl₂. Therefore, the production efficiency and thermal efficiency are largely improved, the reaction temperature is easily controlled, and the method of producing Ti or the Ti alloy through reduction by Ca can be performed further efficiently.

Additionally, in producing Ti or the Ti alloy through reduction by Ca, when the solid-state Ca and electrolytic-bath salt are fed as the Ca source to the molten salt containing CaCl₂ in the reactor vessel, unlike the case in which the solid-state metallic Ca is fed as the Ca source, the solid-state Ca and electrolytic-bath salt are dissolved rapidly and evenly, and the reaction of Ca and the metallic chloride containing TiCl₄ can be caused to proceed evenly in a wide range inside the reactor vessel.

Thus, in the method of producing Ti or the Ti alloy through reduction by Ca, the large effect is obtained by utilizing the solid-state Ca and electrolytic-bath salt as the Ca source.

The invention is made based on study result on the method of producing Ti or the Ti alloy through reduction by Ca, the involvement with the technique of producing Ti or the Ti alloy through reduction by Ca, and the findings (a) to (d). The summary of the invention includes the following Ti or Ti alloy production methods (1) to (3) and pulling electrolysis methods (4) and (5).

(1) A method of producing Ti or a Ti alloy going through a reduction process in which a metallic chloride containing TiCl₄ is caused to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the electrolytic-bath salt, the method is characterized in that the solid-state Ca and electrolytic-bath salt are fed to the reduction process.

(2) A Ti or Ti alloy production method including a reduction process in which a metallic chloride containing TiCl₄ is caused to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the electrolytic-bath salt and an electrolysis process of generating Ca by electrolyzing the electrolytic-bath salt taken out from the reduction process, the method is characterized in that, in the electrolysis process, a solid substance containing Ca and the electrolytic-bath salt is recovered and the solid substance is delivered to the reduction process.

(3) In the Ti or Ti alloy production method of (1) and (2), the recovery of the solid substance containing Ca and the electrolytic-bath salt may be performed by pulling a cathode while the solid substance is caused to adhere to and solidified in a cathode surface. Additionally, when a Ca-containing electrolytic-bath salt containing CaCl₂ is used as the Ca-containing electrolytic-bath salt, desirably the method of producing Ti or the Ti alloy through reduction by Ca proposed by the inventors can further efficiently be performed.

(4) A pulling electrolysis method of recovering a solid-state Ca using a Ca-containing electrolytic-bath salt, the method is characterized in that a electrolytic-bath salt is electrolyzed at a bath temperature of 680 to 900° C., a cathode current density of 0.1 to 200 A/cm², and a voltage of 10V or less, and Ca and the electrolytic-bath salt is recovered in a solid state by pulling a cathode at a pulling rate of 0.05 cm/min or more while a solid substance is caused to adhere to and solidified in a cathode surface. The pulling electrolysis method can be applied as the electrolysis method of recovering Ca from the Ca-containing electrolytic-bath salt in the electrolysis process described in the Ti or Ti alloy production methods (1) and (2).

(5) In the pulling electrolysis method (4), desirably the pulling rate satisfies the following equation (1):

V≧0.0035×t−2.4  (1)

where V: cathode pulling rate (cm/min), and

t: bath temperature (° C.).

Additionally, when a Ca-containing electrolytic-bath salt containing CaCl₂ is used as the Ca-containing electrolytic-bath salt, desirably Ti or the Ti alloy production can further efficiently be performed in the case of adopting the Ti or Ti alloy production methods (1) and (2).

As used herein, “solid-state” defined in the invention shall mean that the solid substance in the cathode surface is apparently in the solid state (including the state in which the surface is wetted during solidification) at a time when the cathode is pulled. Specifically, “solid-state” shall mean both the case in which the whole of the solid substance is in the solid state (i.e., the solidification is completed) and the case in which, although the solid substance is apparently in the solid state, actually the solid substance is partially solidified and the electrolytic-bath salt or the like in the molten state exists in the solid substance.

The Ti or Ti alloy production methods (1) and (2) each is the method in which, in producing Ti or the Ti alloy through reduction of TiCl₄ by Ca in the electrolytic-bath salt, the electrolytic-bath salt taken out from the reduction process is electrolyzed to recover Ca and the electrolytic-bath salt as the solid substance, and the recovered Ca and electrolytic-bath salt are delivered to the reduction process. In the Ti or Ti alloy production methods (1) and (2), heat generation is suppressed in the reduction process to largely improve production efficiency and thermal efficiency, a reaction temperature is easily controlled, and Ti or the Ti alloy can efficiently be produced at low cost.

The pulling electrolysis methods (4) and (5) are the method of recovering Ca by regulating the bath temperature, the cathode current density, the voltage, and the pulling rate of the cathode in predetermined ranges. According to these methods, Ca can be obtained as the solid-state Ca and electrolytic-bath salt at low voltage and high current efficiency, i.e., with relatively small electric power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a schematic configuration of a main part of a calcium furnace for producing Ca by electrolysis of a molten CaCl₂, disclosed in a Germany technical document;

FIG. 2 is a view for explaining a pulling electrolysis method according to the invention;

FIG. 3 is a view illustrating relationships between a cathode pulling rate and current efficiency when the electrolysis method according to the invention is implemented;

FIG. 4 is a view showing relationships between a bath temperature and a cathode pulling rate in the pulling electrolysis; and

FIG. 5 is a view showing a configuration example of an apparatus for producing a metallic Ti through reduction by Ca.

BEST MODE FOR CARRYING OUT THE INVENTION

A Ti or Ti alloy production method according to the invention, a pulling electrolysis method which can be applied thereto, and an optimum production process of the invention in which these methods are combined will be individually described below.

1. Ti or Ti Alloy Production Method

The Ti or Ti alloy production method of the invention is a method of producing Ti or a Ti alloy going through a reduction process in which a metallic chloride containing TiCl₄ is caused to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the electrolytic-bath salt, the method being characterized in that the solid-state Ca and electrolytic-bath salt are fed to the reduction process.

A specific configuration as an example is “a Ti or Ti alloy production method including a reduction process in which a metallic chloride containing TiCl₄ is caused to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the electrolytic-bath salt and an electrolysis process of generating Ca by electrolyzing the electrolytic-bath salt taken out from the reduction process, the method being characterized in that, in the electrolysis process, a solid substance containing Ca and the electrolytic-bath salt is recovered and the solid substance is delivered to the reduction process.

That is, the Ti or Ti alloy production method of the invention is characterized in that the solid substance containing Ca and the electrolytic-bath salt is recovered in the electrolysis process and the recovered solid substance is delivered to the reduction process in the method of producing Ti or the Ti alloy by reducing TiCl₄ by Ca in the electrolytic-bath salt.

A molten salt is used as the electrolytic-bath salt. Usually the molten salt containing CaCl₂ is preferably used. However, the molten salt is not limited to the molten salt containing CaCl₂. Any molten salt can be used as the electrolytic-bath salt, because the reducing reaction by Ca proceeds unless the molten salt having conductivity has extremely small solubility for Ca. There is no limitation in a solid substance recovering method. Ca generated in the electrolysis process is taken out from the electrolytic bath as the solid substance containing the electrolytic-bath salt, and Ca is delivered to the reduction process (namely, Ca is loaded in a reactor vessel in which the reduction process proceeds).

At this point, the whole amount of Ca generated in the electrolysis process may be delivered in the form of the solid substance to the reduction process, or Ca may partially be delivered in the form of the solid substance to the reduction process while the residue may be returned in the form of the CaCl₂ solution Whose Ca concentration is increased. Even in this case, the above effects (such as the improvement of the production efficiency and thermal efficiency and the improvement of the reaction temperature control) are obtained depending on an extent of the delivered solid substance.

Adopting the “solid substance recovery-delivery” method can decrease heat loss to largely improve the thermal efficiency while absorb the heat generation to enhance the production efficiency by utilizing latent heat of fusion possessed by the solid substance containing Ca and CaCl₂. Because cooling capability in the reaction system is increased as a whole, the reaction temperature is easily controlled, and the raw-material loading rate can be enhanced to further efficiently produce Ti or the Ti alloy through the reduction by Ca at low costs. The compact apparatus can also be realized.

In the method of producing Ti or the Ti alloy through reduction by Ca, the reaction rates in the reduction process and electrolysis process are possibly changed during operation (or the reaction rates are inevitably changed). In such cases, an added effect that the solid substance containing Ca and CaCl₂ is retained (temporarily stored) as the Ca source and used according to need can be sufficiently expected.

That is, the solid substance containing Ca and CaCl₂ can act as a buffer in adjusting the reaction rates in the reduction process and electrolysis process. When the CaCl₂ solution whose Ca concentration is increased is kept in a high-temperature state, or when CaCl₂ solution is regained to be in a dissolution state when in use after cooled once, the energy loss becomes extremely large. In addition, because the solid substance containing the high-concentration Ca can be recovered, an amount of Ca transported to the reduction process can be decreased compared with the case in which Ca is returned to the reduction process along with CaCl₂ solution.

In the Ti or Ti alloy production method of the invention, the recovered Ca and the Ca concentration in the solid substance containing the electrolytic-bath salt can be adjusted by controlling the temperature of the Ca-containing electrolytic-bath salt. For example, the Ca concentration in the solid substance can be adjusted to 20% by weight by setting the electrolytic temperature to 720° C., and the Ca concentration in the solid substance can be adjusted to 30% by weight by setting the electrolytic temperature to 800° C.

That is, the Ca concentration in the solid substance containing Ca and the electrolytic-bath salt can be controlled by managing the electrolytic temperature. For example, in the case where a temperature in a reaction field where the reducing reaction is generated is lowered, the electrolytic temperature is lowered to decrease the Ca concentration in the solid substance. On the other hand, in the case where a reaction rate is enhanced in the reduction process, the electrolytic temperature is raised to increase the Ca concentration in the solid substance. Thus, the solid substance containing Ca and the electrolytic-bath salt can be suitably selectively used as the operation situation demands.

2. Pulling Electrolysis Method

As described above, the pulling electrolysis method of the invention is a method characterized in that an electrolytic-bath salt is electrolyzed at a bath temperature of 680 to 900° C., a cathode current density of 0.1 to 200 A/cm², and a voltage of 10V or less, and the electrolytic-bath salt and Ca is recovered in a solid state by pulling a cathode at a pulling rate of 0.05 cm/min or more while a solid substance is caused to adhere to and solidified in a cathode surface.

FIG. 2 is a view for explaining the pulling electrolysis method of the invention. As shown in FIG. 2, an electrolytic-bath salt 2 is retained in an electrolytic tank 1, and a positive electrode 3 and a cathode 4 are provided. When electricity is passed between the electrodes to start the electrolysis, chlorine (Cl₂) is generated at the anode 3 and Ca is deposited at the cathode 4.

At this point, when the cathode 4 is gradually pulled upward as shown by an arrow in FIG. 2, because the temperature is rapidly lowered in an exposed portion above the liquid surface of the electrolytic-bath salt 2, the electrolytic-bath salts adhering to Ca generated (deposited) by the electrolysis start the solidification one after another. On the other hand, because the conductivity is maintained, Ca is continuously deposited while the electrolytic-bath salt is solidified.

The deposition of Ca and the adhesion and solidification of the electrolytic-bath salt near the deposited Ca repeatedly undergo as the cathode 4 is pulled, and the solid-state Ca and electrolytic-bath salt 5 in which Ca and the electrolytic-bath salt are contained in a mixed state are formed downward from the portion which is dipped in the electrolytic-bath salt 2 in starting the electrolysis of the cathode 4.

In the solid-state Ca and electrolytic-bath salt, Ca is dispersed in fine granules, and Ca has an extremely large surface area. Accordingly, in the method of producing Ti or the Ti alloy through reduction by Ca, the solid-state Ca and electrolytic-bath salt have a property of being easily dissolved in the molten salt containing CaCl₂ in the reactor vessel when the solid-state Ca and electrolytic-bath salt are used as the Ca source. For the “state in which Ca and the electrolytic-bath salt are mixed”, a mixing ratio and unbalance of the mixing are not particularly defined. Any solid-state Ca and electrolytic-bath salt may be used as long as they are rapidly and evenly dissolved in the molten salt containing CaCl₂ when fed into the molten salt as the Ca source.

In the electrolysis method of the invention, the electrolytic-bath salt having any composition can be used as long as the temperature of the electrolytic-bath salt can be adjusted within the above-described temperature range and Ca is generated by the electrolysis. Usually a mixture of a halogenated salt of Ca is used. Examples of the mixture of a halogenated salt of Ca includes a binary-system mixed salt such as a calcium fluoride and a calcium chloride and the calcium chloride and a potassium chloride and a ternary-system mixed salt such as the calcium chloride, the calcium fluoride, and the potassium chloride. Because a melting point of the electrolytic-bath salt can be changed by the use of the mixed salt, the electrolytic-bath salt can be selected according to the setting bath temperature.

The bath temperature is set to a range of 680 to 900° C. When the bath temperature is lower than 680° C., the electrolytic generation of Ca is hardly performed due to the excessively low reaction temperature. On the other hand, when the bath temperature exceeds 900° C., it is difficult to obtain high current efficiency (i.e., Ca recovery efficiency) because a dissolution amount of generated Ca in the electrolytic-bath salt is increased.

The cathode current density is set in the range of 0.1 to 200 A/cm². A lower limit of the current density depends on a rate at which Ca generated by the electrolysis is re-dissolved. When the current density is less than 0.1 A/cm², because the rate at which Ca is dissolved in the electrolytic-bath salt is faster than the Ca generation rate, Ca cannot be recovered. On the other hand, the reason why an upper limit of the current density is set to 200 A/cm² is that, when the electrolysis is performed at the current density exceeding 200 A/cm², the voltage cannot be decreased even if the inter-electrode distance is adjusted, and the electric power consumption is increased.

When the cathode current density is set in the range of 0.1 to 70 A/cm², the voltage can be set to 5V or less to largely reduce the electric power consumption, which is desirable. Further, when the cathode current density is set to the range of 10 to 50 A/cm², at least 90% current efficiency can be obtained in addition to the large reduction of the electric power consumption.

That is, the demands of the decrease in voltage and the current efficiency improvement, which conflict with each other, can be satisfied. Accordingly, the above range is the compatible range for the cathode current density, and desirably the commercial operation is performed while the cathode current density is set in the range of 10 to 50 A/cm².

The reason why the voltage is set to 10V or less during the electrolysis is that the increase in electric power consumption is suppressed as much as possible. In the description of the Germany technical document, it is considered that the electrolysis is performed at high voltage (35 to 40V) in order to deposit the metallic Ca. However, the high voltage is not required in the electrolysis method of the invention, because Ca is recovered as the solid substance in which the Ca and the electrolytic-bath salt are mixed. Although the lower limit of the voltage is not particularly determined, it is necessary that the voltage be higher than at least a decomposition voltage (about 3.2V) of the molten CaCl₂ in order that the electrolysis proceeds to deposit Ca.

The cathode pulling rate is set to 0.05 cm/min or more. When the pulling rate is slower than 0.05 cm/min, it is difficult to cause the generated Ca to adhere to the cathode surface. This is because the generated Ca is dissolved and widely spread into the bath.

The upper limit of the pulling rate is not particularly defined. As regulated in the electrolysis method of the invention, when a manipulation for pulling the cathode while the solid substance is caused to adhere to and solidified in the cathode surface is performed, the upper limit of the pulling rate is determined by itself. That is, when the pulling rate is excessively fast, the pulled solid substance has an excessively small sectional area (i.e., becomes excessively thin) and the pulled solid substance is cut in the middle, so that the pulling cannot continuously be performed. In consideration of the restriction on the pulling manipulation, desirably the pulling rate is set to 10 cm/min or less.

In the electrolysis method of the invention, when the pulling rate further satisfies the equation (1), Ca and the electrolytic-bath salt can be recovered at good current efficiency.

FIG. 3 is a view illustrating relationships between the cathode pulling rate and the current efficiency when the pulling electrolysis method of the invention is implemented. FIG. 3 shows electrolysis examples at the voltage of 10V or less and the interelectrode distance of 7 cm or less. In FIG. 3, a mark “♦” and a solid line indicate the case in which bath temperature is set to 720° C. during the electrolysis (electrolysis at 720° C.), the mark “♦” and a broken line indicate the case in which the bath temperature is set to 800° C. (electrolysis at 800° C.), a mark

indicates the case in which a columnar cathode whose diameter is 8 mm is used, a mark “” indicates the case in which a columnar cathode whose diameter is 5 mm is used, and a mark “◯” indicates the case in which a columnar cathode whose diameter is 15 mm is use d.

At this point, the current efficiency is expressed by a ratio (percentage) of the Ca amount in the solid substance (i.e., solid-state Ca and electrolytic-bath salt) of the cathode surface to the Ca deposition amount (theoretical deposition amount) determined from the electricity amount based on a Faraday's law. Because Ca in the solid substance does not include Ca which is dissolved or peeled off after once deposited on the cathode surface, the current efficiency used herein is synonymous with Ca recovery efficiency.

As is clear from FIG. 3, the cathode pulling rate is closely related with the current efficiency, the current efficiency is improved when the pulling rate is increased irrespective of the bath temperature. This is attributed to the fact that, although the generated Ca is partially dissolved and spread in the bath from the neighbor of the cathode, the generated Ca is exposed from the surface of the electrolytic-bath salt before dissolved in the bath by increasing the pulling rate, which allows the dissolution to be suppressed to enhance the Ca recovery efficiency (i.e., current efficiency). In the electrolysis at 800° C., an influence of the shape (diameter in section) of the cathode is not observed as far as the investigation is performed.

In the case of the high bath temperature, the current efficiency is slightly lowered. In the example shown in FIG. 3, the electrolysis at 800° C. is lower than the electrolysis at 720° C. in the current efficiency over the whole range of the pulling rate. This is attributed to the fact that the dissolution amount of generated Ca into CaCl₂ is increased to decrease the Ca recovery amount when the bath temperature becomes higher. Accordingly, in the case where the electrolytic-bath salt is used at a particularly high temperature, desirably the pulling rate is increased to perform the electrolysis on the condition that the current efficiency is enhanced.

As the bath temperature is increased, the Ca concentration is increased in the recovered solid-state Ca and electrolytic-bath salt. For example, the Ca concentration is 30% by weight at 800° C. while the Ca concentration is 20% by weight at 720° C. Although the detailed phenomenon is unknown, this is attributed to the fact that, in the case of the high bath temperature, the electrolytic-bath salt adhering to the cathode surface (deposited Ca surface) during the pulling runs off to be easily separated from Ca before the electrolytic-bath salt is solidified and Ca in the recovered solid substance is condensed.

Accordingly, the Ca concentration can be controlled in the recovered solid-state Ca and electrolytic-bath salt by adjusting the bath temperature, and the Ca concentration can be learned and arbitrarily determined when the solid-state Ca and electrolytic-bath salt are used as the Ca source.

In the equation (1), the relationship between the bath temperature and the pulling rate in which the good current efficiency is obtained is determined based on the relationship among the bath temperature, the cathode pulling rate, and the current density during the electrolysis shown in FIG. 3. In the electrolysis method of the invention, when the equation (1) is satisfied, Ca can efficiently be recovered as the solid-state Ca and electrolytic-bath salt.

FIG. 4 is a view showing a relationship between the bath temperature and the cathode pulling rate in the pulling electrolysis. A portion hatched in FIG. 4 is a region, where the pulling rate is not lower than 0.05 cm/min and the good current efficiency (i.e., good Ca recovery efficiency) expressed by the equation (1). The lower limit of the region is expressed by an equation (V=0.0035×t−2.4, where 700<=t<=900) in the case where both sides of the equation (1) are equal to each other.

In the electrolysis method of the invention, Ca and the electrolytic-bath salt are recovered in the solid state. As described above, “solid-state” means that the Ca and the electrolytic-bath salt are apparently in the solid state. For example, in the case where a large difference is made between the bath temperature and the melting point of the electrolytic-bath salt due to the high bath temperature, sometimes the molten electrolytic-bath salt exists inside the solid substance on the cathode surface. The electrolytic-bath salt adhering to the pulled cathode surface is hardly solidified, and Ca generally has the melting point higher than that of the electrolytic-bath salt. Therefore, Ca is deposited in the form of the solid substance from the beginning, or Ca becomes immediately the solid substance even if Ca is initially in the molten state, so that the unsolidified electrolytic-bath salt is taken into the solid substance.

In the case of the small difference between the bath temperature and the melting point of the electrolytic-bath salt, because the electrolytic-bath salt is easily solidified, the whole of the solid substance on the cathode surface is recovered as the solid state.

Thus, according to the electrolysis method of the invention, Ca can be obtained as the solid-state Ca and electrolytic-bath salt at low voltage and high current efficiency (i.e., with the relatively small electric power consumption). The solid-state Ca and electrolytic-bath salt is extremely effectively used as the Ca source when the method of producing Ti or the Ti alloy through reduction by Ca is implemented.

3. Production Process

A production process into which the pulling electrolysis method of the invention is incorporated in producing Ti or the Ti alloy through reduction by Ca will be described below.

The production process is a method in which solid-state Ca and electrolytic-bath salt recovered by the pulling electrolysis method of the invention is used as Ca caused to react with a metallic chloride containing TiCl₄ in implementing the method of producing Ti or the Ti alloy through reduction by Ca, i.e., the Ti or Ti alloy production method including a reduction process of causing a metallic chloride containing TiCl₄ to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the electrolytic-bath salt and an electrolysis process of generating Ca by electrolyzing the electrolytic-bath salt taken out from the reduction process.

FIG. 5 is a view showing a configuration example of an apparatus for producing the metallic Ti through reduction by Ca. In the example, TiCl₄ is used as the raw material, and the Ca-containing electrolytic-bath salt containing CaCl₂ is used as the Ca-containing electrolytic-bath salt. In the example, a separation process and a chlorination process are included in addition to the reduction process (below-mentioned process proceeding in the reactor vessel 6) and the electrolysis process. In the separation process, the generated metallic Ti is separated and recovered. In the chlorination process, TiCl₄ is produced by utilizing the chlorine (Cl₂) generated through the electrolysis.

Referring to FIG. 5, a reducing agent feed pipe 7 for feeding Ca (in this case, feeding the solid-state Ca and electrolytic-bath salt) which is of a reducing agent is provided in a ceiling portion of a reactor vessel 6. A bottom of the reactor vessel 6 is tapered downward while a diameter of the reactor vessel 6 is gradually reduced in order to promote discharge of produced Ti granules. A Ti discharge pipe 8 is provided in a central portion at lower end of the reactor vessel 6 to discharge the produced Ti granules.

On the other hand, a cylindrical separation wall 9 is disposed inside the reactor vessel 6 while a predetermined gap is provided between the separation wall 9 and an inner surface of a straight body portion of the reactor vessel 6. A molten salt discharge pipe 10 is provided in an upper portion of the reactor vessel 6 to discharge CaCl₂ in the vessel to the side. A raw material feed pipe 11 for feeding TiCl₄ which is of a raw material to Ti is provided in a lower portion of the reactor vessel 6 while piercing through the separation wall 9 to reach a central portion of the vessel.

The molten CaCl₂ solution in which Ca is melted is retained as the molten salt in the reactor vessel 6. A level of the molten CaCl₂ solution is set higher than the molten salt discharge pipe 10 and lower than an upper end of the separation wall 9. In this state of things, a TiCl₄ gas is supplied to the molten CaCl₂ solution located inside the separation wall 9 through the raw material feed pipe 11. This enables TiCl₄ to be reduced by Ca in the molten CaCl₂ solution inside the separation wall 9 to produce the granular metallic Ti in the molten CaCl₂ solution.

The Ti granules produced in the molten CaCl₂ solution located inside the separation wall 9 in the reactor vessel 6 moves downward through the molten CaCl₂ solution and deposited on the bottom of the reactor vessel 6. The deposited Ti granules are appropriately taken out downward from the Ti discharge pipe 8 along with the molten CaCl₂ solution and delivered to a separation process 12.

The molten CaCl₂ solution whose Ca is consumed by the reducing reaction inside the separation wall 9 rises along the outside of the separation wall 9 though the bottom of the separation wall 9, and the molten CaCl₂ solution is discharged from the molten salt discharge pipe 10. The discharged molten CaCl₂ solution is delivered to an electrolysis process 13.

Inside the separation wall 9, Ca is replenished by feeding the solid-state Ca and electrolytic-bath salt to the molten CaCl₂ solution from the reducing agent feed pipe 7.

On the other hand, in the separation process 12, the Ti granules taken out of the reactor vessel 6 along with the molten CaCl₂ solution are separated from the molten CaCl₂ solution. Specifically, the Ti granules are compressed by squeezing the molten CaCl₂ solution. The molten CaCl₂ solution obtained in the separation process 12 is delivered to the electrolysis process 13.

In the electrolysis process 13, a molten CaCl₂ solution 13 b introduced into an electrolytic tank 13 a from the reactor vessel 6 and the separation process 12 are separated into Ca and the Cl₂ gas through the electrolysis.

Ca generated on the side of a cathode 13 d is recovered by the manipulation for pulling the cathode 13 d in the form of solid-state Ca and electrolytic-bath salt 13 e in which Ca and the electrolytic-bath salt are mixed, and the solid-state Ca and electrolytic-bath salt 13 e are returned into the reactor vessel 6 to replenish Ca. The total of Ca may be replenished (fed) with the solid-state Ca and electrolytic-bath salt, or part of Ca may be replenished with the solid-state Ca and electrolytic-bath salt while the residue is replenished with the CaCl₂ solution whose Ca concentration is increased.

The solid-state Ca and electrolytic-bath salt 13 e delivered in the reactor vessel 6 are easily dissolved, so that they are rapidly and evenly dissolved in the reactor vessel. In the case where the molten electrolytic-bath salt exists in the solid-state Ca and electrolytic-bath salt, because the dissolution proceeds further rapidly, the even reaction between TiCl₄ and Ca proceeds effectively in the wide range of the vessel.

Additionally, the solid-state Ca and electrolytic-bath salt 13 e are melted by absorbing the heat generated in association with the reaction between TiCl₄ and Ca, which allows the production efficiency and the thermal efficiency to be largely improved. Because the cooling capability in the reaction system is enhanced as a whole, the reaction temperature is easily controlled, and raw-material loading rate can be enhanced to produce further efficiently Ti or the Ti alloy through reduction by Ca. The latent heat of fusion can maximally be utilized when the whole of the solid-state Ca and electrolytic-bath salt are in the solid state.

In the method of producing Ti or the Ti alloy through reduction by Ca, the reaction rates in the reduction process and electrolysis process are possibly changed during operation (or the reaction rates are inevitably changed). In such cases, an added effect that the solid-state Ca and electrolytic-bath salt are retained (temporarily stored) as the buffer of the Ca source and used according to need can sufficiently be expected.

The Cl₂ gas generated on the side of a positive electrode 13 c in the electrolysis process 13 is delivered to a chlorination process 14. In the chlorination process 14, TiO₂ is chlorinated to produce TiCl₄ in the presence of carbon powders (C). Oxygen which is of a by-product is discharged in the form of CO₂ by simultaneously using the carbon powders (C). The produced TiCl₄ is introduced into the reactor vessel 6 through the raw material feed pipe 11. Thus, Ca and Cl₂ gas which are of the reducing agent are circulated by the circulation of CaCl₂.

As described above, according to the production process, the metallic Ti can continuously be produced only by actually replenishing TiO₂ and C. At this point, the solid-state Ca and electrolytic-bath salt which are recovered by the electrolysis method of the invention can suitably used as the Ca source.

INDUSTRIAL APPLICABILITY

According to the production method of the invention, in producing Ti or the Ti alloy through reduction by Ca, the electrolytic-bath salt taken out from the reduction process is electrolyzed to recover the Ca and electrolytic-bath salt in the form of the solid substance, and the recovered Ca and electrolytic-bath salt is delivered to the reduction process. Therefore, the heat generation in the reduction process is suppressed by utilizing the latent heat of fusion possessed by the solid substance, the production efficiency and the thermal efficiency are largely improved, the reaction temperature is easily controlled, and the raw-material loading rate can be enhanced to efficiently produce Ti or the Ti alloy at low cost. In the pulling electrolysis method of the invention, Ca is recovered while the bath temperature, the cathode current density, the voltage, and the cathode pulling rate are regulated within the predetermined ranges. Therefore, the solid-state Ca and electrolytic-bath salt can be obtained at low voltage and high current efficiency, i.e., with the relatively small power consumption. When the solid-state Ca and electrolytic-bath salt are used as the Ca source in producing Ti or the Ti alloy through reduction by Ca, the solid-state Ca and electrolytic-bath salt are dissolved rapidly and evenly in the reactor vessel, and the excessive heat generated by the reaction of the metallic chloride containing Ca and TiCl₄ is suppressed by melting the Ca and electrolytic-bath salt during the reducing reaction, so that Ti or the Ti alloy can efficiently be produced. 

1. A method of producing Ti or a Ti alloy going through a reduction process in which a metallic chloride containing TiCl₄ is caused to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the electrolytic-bath salt, the method comprising feeding the solid-state Ca and electrolytic-bath salt to the reduction process.
 2. The Ti or Ti alloy production method according to claim 1, wherein a Ca-containing electrolytic-bath salt containing CaCl₂ is used as the Ca-containing electrolytic-bath salt.
 3. A Ti or Ti alloy production method including a reduction process in which a metallic chloride containing TiCl₄ is caused to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the electrolytic-bath salt and an electrolysis process in which Ca is generated by electrolyzing the electrolytic-bath salt taken out from the reduction process, the method comprising: recovering a solid substance containing Ca and the electrolytic-bath salt in the electrolysis process; and delivering the solid substance to the reduction process.
 4. The Ti or Ti alloy production method according to claim 3, wherein the recovery of the solid substance containing Ca and the electrolytic-bath salt is performed by pulling a cathode while the solid substance is caused to adhere to and solidified in a cathode surface.
 5. The Ti or Ti alloy production method according to claim 3, wherein a Ca-containing electrolytic-bath salt containing CaCl₂ is used as the Ca-containing electrolytic-bath salt.
 6. The Ti or Ti alloy production method according to claim 4, wherein a Ca-containing electrolytic-bath salt containing CaCl₂ is used as the Ca-containing electrolytic-bath salt.
 7. A pulling electrolysis method of recovering a solid-state Ca using a Ca-containing electrolytic-bath salt, the method comprising: electrolyzing an electrolytic-bath salt at a bath temperature of 680 to 900° C., a cathode current density of 0.1 to 200 A/cm², and a voltage of 10V or less; and recovering Ca and the electrolytic-bath salt in a solid state by pulling a cathode at a pulling rate of 0.05 cm/min or more while a solid substance is caused to adhere to and solidified in a cathode surface.
 8. The pulling electrolysis method according to claim 7, wherein the pulling rate satisfies a following equation (1): V≧0.0035×t−2.4  (1) where V: cathode pulling rate (cm/min), and t: bath temperature (° C.).
 9. The pulling electrolysis method according to claim 7, wherein a Ca-containing electrolytic-bath salt containing CaCl₂ is used as the Ca-containing electrolytic-bath salt.
 10. A pulling electrolysis method including a reduction process in which a metallic chloride containing TiCl₄ is caused to react with Ca in a Ca-containing electrolytic-bath salt to generate Ti or the Ti alloy in the electrolytic-bath salt and an electrolysis process in which Ca is generated by electrolyzing the electrolytic-bath salt taken out from the reduction process, the Ca being recovered from the Ca-containing electrolytic-bath salt in the electrolysis process, the method comprising: electrolyzing an electrolytic-bath salt at a bath temperature of 680 to 900° C., a cathode current density of 0.1 to 200 A/cm², and a voltage of 10V or less; and recovering Ca and the electrolytic-bath salt in a solid state by pulling a cathode at a pulling rate of 0.05 cm/min or more while a solid substance is caused to adhere to and solidified in a cathode surface.
 11. The pulling electrolysis method according to claim 10, wherein the pulling rate satisfies a following equation (1): V≧0.0035×t−2.4  (1) where V: cathode pulling rate (cm/min), and t: bath temperature (° C.).
 12. The pulling electrolysis method according to claim 10, wherein a Ca-containing electrolytic-bath salt containing CaCl₂ is used as the Ca-containing electrolytic-bath salt. 